Fixed strict error "Declaration of *::bind() should be compatible with Monad::bind()"

Original code throws strict error "Declaration of Maybe::bind() should be compatible with Monad::bind($function, array $args = Array)" when "error_reporting = E_STRICT" is set in php.ini.

Avoid this issue, base Monad class should not implement bind function.
But Monad class can implement magic method, and bind method implement in it.